DE69223387T2 - MULTIPLE-USE DYE-CONTAINING LAYER FOR THERMAL TRANSFER PRINTING - Google Patents

Description

The present invention relates to the use of a reusable thermal transfer printing ribbon in an indirect thermal transfer process.

Previously, a reusable thermal transfer printing ink ribbon or sheet was known in which a heat-meltable thermal transfer printing ink layer is provided on a substrate, which contains a heat-meltable substance and a pigment as the main components. Such a printing ink ribbon is designed in such a way that the thermal transfer printing ink layer is transferred to a receiver in portions in terms of its thickness each time the printing ink layer is heated by a thermal head.

As far as a thermal transfer ribbon of this type is used in printing with a common thermal transfer method (hereinafter referred to as "direct thermal transfer method") in which a thermal transfer ribbon is placed on a receiver and printing ink of the ribbon is directly transferred to the receiver, such a thermal transfer ribbon could be reused several times.

EP-A-0 351 794 describes a printing ink ribbon comprising a support having thereon a heat-fixable ink containing an acid wax and a metal oxide.

EP-A-0 076 044 describes a thermal transfer printing ink ribbon comprising a carrier coated with an ink layer containing a pigment and a binder consisting of a thermoplastic resin and a plasticizer consisting of one or more waxes.

US-A-4-617 224 describes a thermal transfer recording medium which comprises a support, a first dye layer containing a dye and a resin applied to the support, and a further dye layer containing a dye and a heat-meltable wax substance.

JP-A-59-24693 describes a printing ink for a heat-sensitive ribbon which can be applied to a substrate without heating and is suitable for multi-coloured coatings by using a composition prepared by dispersing fine particulate wax and a dye in a volatile dispersion medium as the printing ink for the transfer type heat-sensitive ribbon.

Recently, on the other hand, a thermal transfer method called "indirect thermal transfer method" has been proposed.

This indirect thermal transfer method uses a device as shown in Fig. 1. In Fig. 1, numeral 10 denotes a rotary transfer drum whose surface is made of an elastic material having good release properties such as silicone rubber or fluorine-containing rubber. Numeral 11 denotes a recording part which is arranged so that a thermal head 13 can press a thermal transfer printing ribbon 12 against the transfer drum 10. The printing ribbon 12 is moved in the direction indicated by an arrow while the transfer drum 10 rotates for recording. Numeral 14 denotes a transfer part which is arranged so that a receiver 15 is pressed against the transfer drum 10 by means of a pressure roller 16. During printing, the receiver 15 is moved in the direction indicated by an arrow.

The thermal head 13 heats the thermal transfer ribbon 12 so that the ink is softened or melted thereon, which is then transferred to the surface of the transfer drum 10. While the transfer drum 10 and the ribbon 12 are thus moved in the directions indicated by the respective arrows, the softened or melted ink is transferred to the transfer drum 10 so that the ink image 17 is formed thereon. The ink image 17 is transported to the transfer part 14 while the transfer drum 10 is rotating, pressed against the receiver 15 there, and transferred to the receiver 15 to finally form the ink image 18.

The transfer drum 10 is always heated to about 60º to about 80ºC in order to facilitate the transfer of the color image thereon to the receiver 15.

The present inventors attempted to perform multiple printing in such an indirect thermal transfer method by using a reusable thermal transfer ink ribbon of the above-mentioned type. As a result, it was found that multiple printing was difficult due to poor transferability of the ink to the transfer drum.

In view of the foregoing, an object of the present invention is to provide a thermal transfer printing ribbon capable of multiple printing even in the indirect thermal transfer method.

DESCRIPTION OF THE INVENTION

The present invention relates to the use of a reusable thermal transfer printing ribbon as defined in claim 1; preferred embodiments thereof are defined in claims 2 to 5.

SHORT DESCRIPTION OF THE DRAWING

Fig. 1 is an explanatory view showing an example of a printer for use in an indirect transfer method.

In the thermal transfer printing ribbon having the above structure, both the transferability of the thermal transfer printing ink to a transfer drum made of silicone rubber or the like which is heated as described above and the releasability of the printing ink on the transfer drum from the transfer drum to a receiver are satisfactory, and hence good multi-printing performance is achieved. Although the reason for this has not yet been found, it is conceivable that by incorporating the wax compound having a polar group into a thermal transfer printing ink, the wettability of the printing ink with respect to the donor surface of the transfer drum is improved by means of the polar group of the wax, with the result that the transferability and releasability of the printing ink are brought into a balanced relationship with each other. On the other hand, the use of a wax that does not have a polar group does not ensure good transfer of the ink to the transfer drum, resulting in an incomplete or disintegrated print image.

The thermal transfer ink layer according to the present invention contains a wax compound having a polar group as defined in claim 1 and a pigment as main components and optionally contains a heat-fusible resin if necessary.

Examples of the polar group of the wax compound include -COOH, -OH, -NH₂, -NHR¹, -NR²R³, -COOR⁴, -(CH₂CH₂O)nH, wherein n is an integer of 1 to 10, -SO₃H, -OCONHR⁵, R⁹(NHCOO-)m, wherein m is 2 or 3, -CONH₂, -CONHR⁶, -CONR⁷R⁸.,

In these groups, R1, R2, R3, R4, R5, R6, R7 and R8 are independently a monovalent organic group such as an alkyl group (preferably having 1 to 6 carbon atoms), an aralkyl group (preferably having 7 or 8 carbon atoms) and an aryl group (preferably having 6 to 12 carbon atoms) and the like. These groups may each be substituted with a lower alkyl group or the like. R9 is a divalent or trivalent organic group such as an alkylene group (preferably having 1 to 10 carbon atoms), an arylene group (preferably having 6 to 13 carbon atoms) and the like. These groups may each be substituted with a lower alkyl group or the like.

Preferred examples of the wax compound according to the present invention include those each having the above-mentioned polar group in a linear or branched aliphatic carbon (preferably having 30 to 150 carbon atoms, more preferably 30 to 70 carbon atoms).

Although the wax compound must have at least one polar group on its carbon chain, the content of the polar group is preferably 2 to 30%, more preferably 3 to 30%, and most preferably 3 to 10%, based on the number of carbon atoms of the carbon chain. The wax compound may have one type of polar group, or two or more types thereof.

The wax compound is selected from an oxidized monocrystalline wax and wax compounds each of which has the polar group of -OCONHR⁵ or R⁹(NHCOO-)m.

Examples of the above heat-fusible resin include ethylene resins such as ethylene-vinyl acetate copolymer resin (vinyl acetate content: 10 to 40% by weight), ethylene-ethyl acrylate copolymer resin (ethyl acrylate content: 10 to 40% by weight) and ethylene-acrylic acid copolymer resin, diene resins such as styrene-butadiene copolymer resin and 1,2-polybutadiene resin, acrylic ester resins such as acrylic esters and methacrylic esters, polyamide resins, polyester resins, polyurethane resins, rosins, hydrogenated rosins, hydrogenated rosin esters, α-pinene resins, terpene resins, coumarone-indene resins, ketone resins, maleic acid resins and phenol resins. These heat-fusible resins may be used singly or in combination of two or more kinds thereof.

Examples of the above pigment include carbon black, aniline black, perylene black, naphthol yellow S, Hansa yellow 5G, benzidine yellow, quinoline yellow lake color, permanent red F5R, brilliant carmine 6B, pigment scarlet 3B, rhodamine lake color B, Victoria blue lake color, metal-free phthalocyanine blue and phthalocyanine blue.

A dispersant, such as a surfactant, may be incorporated into the thermal transfer ink layer to improve the dispersion of the pigment.

Table 1 below shows preferable ranges for the respective contents of the above-mentioned components of the thermal transfer ink layer used in the present invention, the ranges which ensure transferability to a transfer drum, releasability from the transfer drum to a receiver, adhesion to the receiver such as paper, and multiple transferability. Table 1

(Note) *: The content is based on the total amount of solid components in the ink layer.

If the content of the wax compound is less than the above range, the transferability to the transfer drum becomes poor, while if it is above the range, the multiple transferability becomes insufficient, subjecting the transfer drum to considerable contamination (caused by friction of the ink ribbon on the transfer drum).

The resin is preferably mixed with other ingredients to improve the sharpness of the printed images. The desired improvement in sharpness cannot be expected with a resin content of less than 5% by weight. On the other hand, a resin content that is above the above-mentioned range leads to poor transferability to the transfer drum.

A pigment content above the above-mentioned range results in poor transferability to the transfer drum, while on the other hand the density of the printed image decreases undesirably if it is below the above-mentioned range.

In order to ensure good sharpness of the printed images and to prevent the transfer drum from being stained or soiled, the melt viscosity of the thermal transfer ink layer is preferably in a range of 0.1 to 500 Pa.s (1 to 5000 poise), more preferably 10 to 100 Pa.s (100 to 1000 poise) (the melt viscosity is the melt viscosity at a temperature 30ºC higher than the melting point or softening point of the ink and the value measured with a rheometer manufactured by Rheology Co., Ltd.; hereinafter the same applies). A melt viscosity below the above range easily produces disintegrated printed images and contamination, while on the other hand, if it is above the above range, the releasability to the receiver becomes poor.

Although the melting point or softening point of the ink layer is not particularly limited as long as it is not lower than the temperature of the heated transfer drum, it is preferably set within a range of 1º to 30ºC higher than that temperature.

The thermal transfer ink layer can be formed by dispersing or dissolving each of the above components in a suitable organic solvent to prepare a coating liquid, and applying the coating liquid to a substrate using a suitable coating device such as a roller coating device, an anilox roller coating device, a reverse coating device or a bar coating device, followed by drying.

The amount of the ink layer applied is suitably about 5 to about 20 g/m² in terms of the solid content thereof.

As the base in the present invention, polyester films and polyamide films as well as various plastic films which are usually used as a base film for printing ink ribbons of this type can be used. When such plastic films are used, it is desirable to prevent the base from sticking to the thermal head by providing a well-known anti-stick layer on the back side (the side in sliding contact with the thermal head) of the base. The anti-stick layer is made of one or more various heat-resistant resins which enhance the sliding effect, including silicone resin, fluorine-containing resin, nitrocellulose resin, resins modified with the above-mentioned resins and mixtures of the above-mentioned heat-resistant resins with a lubricant. The base and/or the anti-stick layer may contain an antistatic agent or the like. Furthermore, the base may also be a thin sheet of paper having a high density such as a capacitor paper. The thickness of the base is preferably about 1 to about 9 µm, in particular about 2 to about 4.5 µm, in order to ensure good heat conduction.

In multi-printing according to the indirect thermal transfer method using the thermal transfer ribbon, sharp printed images can be obtained using the same part of the ribbon regardless of the type of receiver. Various materials including various types of paper, plastic films, various types of textiles and the like can be used as the receiver.

BEST WAY TO IMPLEMENT THE INVENTION

Next, the present invention will be described by means of examples and comparative examples.

example 1

A thermal transfer ink layer was formed by applying a coating liquid of the following formula to a 4.5 µm thick polyester film having a 0.1 µm thick non-stick layer of a silicone-modified urethane resin formed on the back thereof, followed by drying. The obtained thermal transfer ink layer had a coating amount of 8 g/m² after drying, a softening point of 90°C and a melt viscosity of 30 Pa.s (300 poise) (at 120°C).

Comparison example 1

A thermal transfer ink ribbon was prepared in the same manner as in Example 1 except that paraffin wax (melting point: 70ºC) was used instead of Cardis 320. The softening point and melt viscosity of the ink were 70ºC and 90 Pa.s (900 poise) (at 100ºC), respectively.

Comparison example 2

A thermal transfer ink ribbon was prepared in the same manner as in Example 1 except that microcrystalline wax (melting point: 70ºC) was used instead of Cardis 320. The softening point and melt viscosity of the ink were 70ºC and 80 Pa.s (800 poise) (at 100ºC), respectively.

Example 2

A thermal transfer ink ribbon was prepared in the same manner as in Example 1 except that the following ink liquid was used for coating. The softening point of the ink was 73°C.

The above urethane wax was prepared in the following manner. Polyethylene wax (Bareco 50 Polywax, manufactured by Petrolite Corporation, weight average molecular weight: 500, average number of carbon atoms: 37.0, softening point: 86ºC) in the molten state was subjected to liquid phase oxidation using air in the presence of a catalyst to obtain a secondary alcohol mixture (hydroxyl group: 83.0 mg KOH/g, average number of hydroxyl groups per molecule: 0.740), which was then reacted with tolylene diisocyanate (95% of the equivalent amounts of the hydroxyl numbers of the secondary alcohol) to obtain the urethane wax.

Each of the thus obtained ink ribbons was subjected to a printing test using the indirect thermal transfer device shown in Fig. 1 and then evaluated on the following items. The transfer drum as used in the test was coated with silicone rubber and heated to 70°C. The receiver as used here was plain paper.

(i) Transferability

The transferability of the ink to the transfer drum was evaluated based on the following criteria:

3... a predetermined amount of ink is transferred to the transfer drum;

2... an insufficient amount of ink is transferred to it;

1... a small amount of printing ink is transferred to it.

(ii) Density of the printed image

The OD value was determined for the image printed on the receiver paper.

(iii) Sharpness of the printed image

The sharpness of the printed image on the receiving paper was assessed with the naked eye.

3... sharp

2... a bit blurry

1... blurred

(iv) Contamination

The degree of contamination on the surface of the transfer drum was judged by the naked eye, with the contamination resulting from the friction between the transfer drum and the printing ribbon.

3... no contamination

2... slight contamination

1... significant contamination

(v) Untransferred ink

The extent to which the image on the transfer drum was not transferred to the receiver paper was assessed by eye.

3... No untransferred ink

2... Little untransferred ink

1... A lot of untransferred ink

The results of the above test are shown in Table 2. Table 2

Claims (5)

1. Use of a reusable thermal transfer printing ink ribbon which comprises a base and a thermal transfer printing ink layer provided on the base, the printing ink layer being transferable in portions with respect to its thickness, the thermal transfer printing ink layer comprising 20 to 80% by weight of a wax compound having a polar group, 0 to 20% by weight of a heat-meltable resin, 15 to 50% by weight of a pigment and 0 to 5% by weight of a dispersant and the wax compound having a polar group comprising at least one representative selected from the group consisting of an oxidized microcrystalline wax, a wax compound having a polar group of the formula -OCONHR&sup5; wherein R⁵ is a monovalent organic group, and a wax compound having a polar group of the formula R9(NHCOO-)m wherein R9 is a divalent or trivalent organic group and m is 2 or 3, is selected for a thermal transfer printing process comprising the steps of: heating a reusable thermal transfer ink ribbon by means of a thermal head to transfer the ink of the ink ribbon to the surface of a transfer drum, thereby forming a print image, the transfer drum being heated to a predetermined temperature and the surface having a release property, transferring the ink image formed on the transfer drum to a receiver, and repeating the foregoing steps using the same reusable thermal transfer ink ribbon. 2. Use according to claim 1, wherein the thermal transfer ink layer comprises 40 to 70% by weight of the wax compound having a polar group, 5 to 20% by weight of a heat-meltable resin and 25 to 40% by weight of the pigment. 3. Use according to claim 1, wherein the transfer drum is heated to a temperature of 60 to 80°C. 4. Use according to claim 1, wherein the surface of the transfer drum comprises a silicone rubber or a fluorine-containing rubber. 5. Use according to claim 1, wherein the thermal transfer ink layer has a melting point or softening point not lower than the surface temperature of the heated transfer drum and a melt viscosity in a range of 0.1 to 500 Pa.s (1 to 5000 poise) at a temperature higher than the melting point or softening point of the thermal transfer ink layer by 30°C.